The photoinduced carrier lifetime is used as one of indicators for evaluating internal defects within the semiconductor substrate. The term “photoinduced carrier lifetime” means the lifetime of the photoinduced carriers (i.e., the minority carriers) generated in the semiconductor substrate by light irradiation.
A μ-PCD (microwave photoconductive decay) method (see, for example, Non-patent document 1) is known as a first example of a method and device for measuring the photoinduced carrier lifetime. In such a method, a laser is pulse-irradiated onto a semiconductor substrate for extremely short time in a state where a microwave is irradiated onto the semiconductor substrate. At this time, the reflectivity of the microwave irradiated onto the semiconductor substrate changes depending on the density of the carriers induced by the laser pulse. Thus, the effective lifetime of the photoinduced carriers of the semiconductor substrate (referred to as “effective lifetime” hereinafter) can be obtained by measuring the change of the reflectivity with time.
A QSSPC (quasi steady state photoconductivity) method (see, for example, Non-patent document 2) is known as a second example of the method and device for measuring the photoinduced carrier lifetime of a semiconductor substrate. In such a method, an inductance coil is disposed to face a semiconductor substrate, to emit RF frequency radiation. Further, a light is pulse-irradiated onto the semiconductor substrate for extremely short time. At this time, an electromagnetic wave of a RF frequency is reflected by the carriers induced by the light pulse. The photoinduced carrier effective lifetime of the semiconductor substrate can be obtained by measuring the change of the reflected wave with time as the change of the currency flowing through the coil.
Further, a microwave optical interference absorption method (see, for example, Non-patent document 3) is known as a third example of the method and device for measuring the photoinduced carrier lifetime of a semiconductor substrate. In such a method, a microwave interferometer formed of a waveguide is inserted into a semiconductor substrate, and continuous light is irradiated onto the semiconductor substrate in a state where a microwave is irradiated onto the semiconductor substrate. At this time, since the microwave is absorbed by the carriers induced by the irradiation of the continuous light, the photoinduced carrier effective lifetime can be obtained by measuring the reduction of the microwave transmittance.